An HNB (Home Node B) is a small-sized radio base station which can be installed within end-user's premises or a similar environment, and connects a UE compatible with a UMTS system to a core network of a mobile operator through a PDN (Public Data Network) such as a broadband IP (Internet Protocol) backhaul.
An HNB-GW (Gateway) accommodates a plurality of HNBs connected through the PDN, and relays traffic between each HNB and communication nodes such as an MSC (Mobile Switching Centre), an SGSN (Serving GPRS (General Packet Radio Service) Support Node), an MGW (Media Gateway) and a GGSN (Gateway GPRS Support Node), which form the core network.
An NB (Node B) is a radio base station which is installed outdoors or a similar environment by the mobile operator, and connects the UE compatible with the UMTS system to the core network. The coverage of a cell (generally, referred to as “macro cell”) formed by the NB is large, so that the number of UEs which can be accommodated in the cell is large. On the other hand, the coverage of a cell formed by the above-mentioned HNB is much smaller than that formed by the NB. Therefore, this cell is generally referred to as “femto cell”.
An RNC (Radio Network Controller) is a control device which accommodates a plurality of NBs and controls radio resources between each NB and the UE. 3GPP has defined RRC (Radio Resource Control) protocol for controlling the radio resources. This RRC protocol is terminated by the RNC and the UE.
Note that details of the HNB, the HNB-GW, the UE, the NB, the RNC, the MSC, the MGW, the SGSN, the GGSN and the PDN have been defined in various specifications of 3GPP (see TS 22.220, TS 25.467, TS 23.002, TS 23.060, TS 25.310 and the like).
In the UMTS, handover between cells is triggered by a Measurement Report message notified from the UE to the RNC. Specifically, the RNC instructs the UE to measure radio quality of a neighboring cell by using a control message in compliance with the RRC protocol (hereinafter, this message will be sometimes referred to as “RRC message”). When the RNC determines that the radio quality of the neighboring cell is higher based on a measurement result notified by the Measurement Report, the RNC instructs the UE to perform the handover.
PSCs (Primary Scrambling Codes) are assigned to respective cells. The UE identifies each cell by using the PSC. The RNC indicates the PSC to the UE to instruct which cell is to be measured for radio quality. The PSCs represent 512 different values, and are assigned so as not to overlap between neighboring cells. The RRC message can designate up to 32 neighboring cells using the same frequency as a cell on which the UE camps and up to 32 neighboring cells using frequencies different from that used in the cell on which the UE camps, upon instructing the UE to measure the quality.
A radius of the macro cell generally extends from several hundred meters to several kilometers. On the other hand, generally a radius of the femto cell is from several meters to several tens of meters, and is set within the range of a house, a company or the like. Moreover, a certain limited number of PSCs among the 512 PSCs are assigned for dedicated use in femto cells, and thus the assigned PSC is shared between femto cells. In other words, there may be a lot of femto cells to which the same PSC is assigned under the coverage of a certain macro cell.
In this case, the RNC cannot determine which femto cell is measured by the UE for radio quality, even if the RNC causes the UE to measure the radio quality of the femto cell and the Measurement Report message notifies about the result of the measurement. As a result, there is caused a problem that the RNC cannot uniquely identify the femto cell serving as a handover destination, and thus hands the UE over to the wrong femto cell even if a handover procedure is performed.
In order to address this problem, there has been studied in 3GPP Release 9, a method of reporting the Measurement Report message with an ID of the measured cell.